Recording accelerations
gives the naval architect actual numbers that describe the motion, which
he can use not only to compare this design objectively to his previous
ones, but also to international standards for ergonomics. Or he can
use them to optimize the structural design, which is especially important
for weight control in higher performance craft.

The last two of types
of tests can be called detail tests, although not everyone we interviewed
deems them necessary. One detail test that Sarin uses involves determining
the ideal spot to position underwater components, such as bow thrusters,
rudders, and propeller tunnels. Wool tufts are attached to the hull
near the component of interest, and the boat is towed through the water
while underwater observations are made of both the direction and motion
of the tufts. Sarin says, "The objective is to eliminate any `surprises'
that could be detrimental to the overall performance; i.e. irregular
flow patterns resulting in additional resistance and/or `dirty'
water flow into the props, etc."

The last test is the
wake wheel test. For this, a series of special, different-size propellers,
called wake wheels, are installed in place of the standard props. The
wheels' rotational speed is measured as the model is towed, which
gives an estimate of how fast the water is at the location of the propeller.
Because it is usually slower than the speed of the boat, this is a factor
in correctly pitching the propeller.

So what does all this
cost, and what is it relative to fixing the problems it can prevent?
According to Stensgaard, model testing can range from $20,000 to $30,000,
depending on the extent of the trials, with a large portion of that
being construction of the model. We also asked Ed Hageman for the usual
fixes for these problems, and then asked yards like Rybovich Spencer
and Knight & Carver for ballpark figures for doing the work. If
the boat doesn't reach the desired speed, or if there are spray
or ride problems, hull modifications can be attempted. This might mean,
from simplest to most complicated, adding ballast, modifying the spray
rails, adding a wedge or extending the bottom to change the running
trim, adding a bulb to trim the bow up, or extending the cockpit to
trim the bow down. Prices can range from $5,000 to $200,000 and higher,
depending on the size of the boat and complexity of work. Increasing
the speed by empowering with larger engines runs in the $30,000 to $100,000
range, plus $200,000 and up for new machinery, including larger shafts,
gears, etc.

Of course, none of
this takes into account cost overruns, which are common, and frustration.
Hageman bluntly concludes, "Often, the problem is so fundamental
that it is economic nonsense to make the change. In which case, live
with it or sell it."

So it would seem cost-effective
to model test once the cost of repairs equals the cost of testing. Sarin
sums up, "It is the builder's and owner's cheapest
form of insurance that the yacht they are building will perform up to
their expectations."

Chris Martin is an
associate member of the Society for Naval Architects and Marine Engineers.